In recent years, lithium secondary batteries have widely been used as driving power supplies for mobile electronic devices and communication devices. In such a lithium secondary battery, in general, a carbon material capable of inserting and extracting lithium is used as a negative electrode, and a composite oxide of transition metal and lithium such as LiCoO2 etc., is used as a positive electrode to provide the secondary battery with high potential and high discharge capacity. With increase of functions of the electronic devices and communication devices, batteries with higher capacity have been in demand.
To realize a high capacity lithium secondary battery, for example, the battery capacity can be increased by increasing a volume of the positive and negative electrodes contained in a battery case, and reducing empty space except for space occupied by the electrodes in the battery case. Further, the battery capacity can be increased by applying a mixture paste made of a material of the positive or negative electrode to a current collector core, drying the paste to form an active material layer, and pressing the active material layer at high pressure to be compressed to a predetermined thickness, thereby increasing a filling density of the active material.
When the filling density of the active material in the electrode increases, it would be difficult to penetrate a nonaqueous electrolyte, which is injected in a battery case and has a relatively high viscosity, into small gaps in an electrode group formed by winding or stacking the positive and negative electrodes at high density with a separator interposed therebetween. Accordingly, it requires a long time to impregnate the electrode group with a predetermined amount of the nonaqueous electrolyte. Further, with an increased filling density of the active material of the electrode, porosity of the electrode is reduced, thereby making penetration of the electrolyte into the electrode group difficult. Therefore, impregnation of the electrode group with the nonaqueous electrolyte is greatly impaired, thereby varying the distribution of the nonaqueous electrolyte in the electrode group.
To overcome this disadvantage, grooves for guiding the nonaqueous electrolyte are formed in a surface of a negative electrode active material layer along a penetrating direction of the nonaqueous electrolyte to allow the nonaqueous electrolyte to penetrate into the whole part of the negative electrode. When the width or depth of the grooves is increased, the impregnation can be done in a short time. However, this reduces the amount of the active material, and therefore, charge/discharge capacity may decrease, or a reaction between the electrodes may become nonuniform, thereby deteriorating battery characteristics. Taking these into consideration, a method for setting the width and depth of the grooves to predetermined values has been proposed (see, e.g., Patent Document 1).
However, the grooves formed in the surface of the negative electrode active material layer may cause break of the electrode when the electrode is wound to form the electrode group. Therefore, a method for preventing the break of the electrode while improving the impregnation has been proposed. In this method, the grooves are formed in the surface of the electrode to form an inclination angle with a longitudinal direction of the electrode in order to distribute tensile force applied in the longitudinal direction of the electrode when the electrode is wound to form an electrode group. This can prevent the break of the electrode (see, e.g., Patent Document 2).
Another method has also been proposed, although it is not intended to improve the impregnation with the electrolyte. In this method, a porous film having convex portions partially formed on a surface facing the positive or negative electrode is provided for the purpose of alleviating overheat caused by overcharge. Accordingly, a larger amount of the nonaqueous electrolyte is held in gaps between the convex portions of the porous film and the electrode than in the other parts, thereby inducing an overcharge reaction in the gaps in a concentrated manner. This can alleviate the overcharge of a battery, and can alleviate the overheat due to the overcharge (see, e.g., Patent Document 3).
In a lithium secondary battery which has achieved high capacity in the above-described manner, for example, the separator may be damaged by a foreign matter that enters the battery for some reason, and an internal short circuit between the positive and negative electrodes may occur. In this case, a current intensively flows through the short circuited portion, thereby causing abrupt heat generation. This may cause decomposition of the positive and negative electrode materials, or boiling or decomposition of the electrolyte, thereby generating gas etc. A solution to these disadvantages derived from the internal short circuit has been proposed, in which a porous protective film is formed to cover the surface of the negative or positive electrode active material layer to reduce the internal short circuit (see, e.g., Patent Documents 4 and 5).    Patent Document 1: Japanese Patent Publication No. H09-298057    Patent Document 2: Japanese Patent Publication No. H11-154508    Patent Document 3: Japanese Patent Publication No. 2006-12788    Patent Document 4: Japanese Patent Publication No. H07-220759    Patent Document 5: Pamphlet of International Patent Publication No. 2005/029614